The present invention relates to optical pumped lasers the active medium of which is composed of a slab. Lasers of this type have a solid active medium having at least two carefully polished parallel faces between which the light beam is propagated along a zig-zag path due to the total reflection from these faces. The thermal energy arising during the laser process must be removed from the faces of the slab. The optical pumped energy needed to produce the laser effect can be introduced into the slab from any direction, provided that it is distributed in homogenous manner, or that non-homogeneous distribution of this energy in the laser medium is eliminated by appropriate guiding of the laser beam. The zig-zag path resulting from total reflection of the beam from the faces is obtained by suitable choice of this type and position of the end faces of the slab in relation to the optical axis thereof and the optical axis of the resonator. Moreover, these end faces can in themselves constitute the resonator. The construction of lasers of this type poses a difficult cooling problem since currently, in the best possible case, only a few percent of the energy supplied to the slab are transformed into energy emitted from the laser beam. This problem is all the more difficult to resolve since the previously proposed solutions cause incompatibility in respect of the liquids used to remove excess heat developed in the laser slab.